With Vim, each filetype has its own set of rules for syntax highlighting (mostly contributed by third-party maintainers). A quick grep through the `/usr/share/vim/vim82/syntax` directory shows the most popular set of such strings that are to be syntax highlighted by Vim are:
I guess these strings are most likely a mix of popular idioms for programming languages or particular to the habits/culture of the individal contributors to the syntax files.
